1. Field of the Invention
This invention relates to a fuel injection amount control system for internal combustion engines, for controlling a fuel injection amount to be supplied to the engine.
2. Prior Art
Conventionally, a fuel injection amount control system for internal combustion engines is known, wherein a physical model is constructed which contemplates a behavior of fuel adhering to the wall surface of the intake pipe when fuel injected into the intake pipe is drawn into the cylinder of the engine, to thereby compensate for transfer delay of fuel adhering to the intake pipe.
According to the known system, provided that the amount of fuel to be injected is represented by TOUT, a desired amount of fuel to be drawn into the cylinder (required fuel amount) by Tcyl, the ratio of a fuel amount to be directly drawn into the cylinder to the whole amount of injected fuel, i.e. the direct supply ratio by A, an amount of fuel adhering to the intake pipe wall surface by Fw, the ratio of a fuel amount drawn into the cylinder by evaporation to the whole amount Fw of the adhering fuel by B, respectively, an amount of fuel adhering to the intake pipe out of the whole amount of injected fuel TOUT is expressed as (1-A).times.TOUT, and an amount of fuel actually drawn into the cylinder as (A.times.TOUT+B.times.Fw). The fuel amount (A.times.TOUT+B.times.FW) corresponds to the desired fuel amount Tcyl to be drawn into the cylinder, and therefore the fuel injection amount to be injected can be calculated by the use of the following equation (1): EQU TOUT=1/A.times.TCYL-B/A.times.Fw (1)
where the desired fuel amount Tcyl to be drawn into the cylinder is determined by parameters indicative of operating conditions of the engine, such as engine rotational speed NE and intake pipe absolute pressure PBA. The direct supply ratio A and the evaporation ratio B are retrieved from maps, based on load on the engine and engine coolant temperature Tw.
However, the above-mentioned conventional fuel injection amount control system has the following inconvenience, and therefore still requires further improvement. More specifically, the direct supply ratio A and the evaporation ratio B, which are parameters indicative of the fuel transfer delay representing fuel adherence characteristics of the interior of the intake pipe, are functions of operating parameters of the engine, which mainly represent dynamic characteristics of fuel exhibited when the engine is in a steady operating condition (i.e. when the NE and PBA values are constant), and moreover, these parameters numerically represent only dynamic characteristics of fuel exhibited when the fuel injection amount changes in a continuous manner.
Therefore, when the engine is operating in a transient state where the fuel injection amount is discontinuous, such as at the start of the engine, immediately after the start of the engine, and immediately after recovery from interruption of fuel supply (fuel cut), and a state where the operating condition of the engine largely changes, the fuel injection amount calculated based upon the parameters for the steady operating condition cannot be stably converged to the required fuel amount Tcyl or can even diverge from the latter, so that the air-fuel ratio A/F of a mixture supplied to the engine largely deviates from a desired value. As a result, the engine suffers from degraded drivability and degraded exhaust emission characteristics.